Polyesters represented by polyethylene terephthalate (PET), polybutylene terephthalate (PBT), and polyethylene naphthalate (PEN) and the like are excellent in mechanical properties and chemical properties and have been used in a wide range of fields, for example, as fibers for clothes and industrial materials; films and sheets for packaging and industrial applications; molded articles such as bottles and engineering plastics.
With respect to polyesters including an aromatic dicarboxylic acid and an alkylene glycol as main constitutional components as representative polyesters, for example in the case of a polyethylene terephthalate (PET), it has been produced industrially by, for example, a polycondensation method that includes performing esterification or transesterification of terephthalic acid or dimethyl terephthalate with ethylene glycol to produce bis(2-hydroxyethyl) terephthalate, and polycondensing this with a catalyst at a high temperature in vacuum.
As a polyester polymerization catalyst to be used in polycondensation of a polyester, technologies of using an alkali metal compound together with an aluminum compound have been disclosed (see, for example, Patent Documents 1, 2, 3, 4 and 5). The use of the above-mentioned polycondensation catalyst can polymerize a polyester having a sufficiently satisfactory degree of polymerization in a relatively good efficiency and therefore it can be an economically advantageous method. Patent Document 3 discloses a technology of improving color tone by adding a nitride, a boride, and a carbide of a specific transition metal element in addition to high catalytic activity of a polyester. Patent Document 4 discloses a technology of improving color tone by adding a phosphorus compound in addition to aluminum and an alkaline earth metal or an alkali metal. Patent Document 5 discloses a technology of improving color tone by adding a magnesium compound or a specific organic phosphorus compound in addition to aluminum and an alkali metal compound.
On the other hand, technologies of forming a polyester polycondensation catalyst having sufficient catalytic activity by adding a specific phosphorus compound to an aluminum compound are also publicly known (see, for example, Patent Documents 6 and 7). It has been reported that the use of the above-mentioned polyester polycondensation catalyst can afford a polyester excellent in heat stability.
Polyesters for films have some problems that when films are superimposed, the films are stuck to each other to cause so-called blocking, or in processing a rolled film, defects such as scratches are generated due to poor slipping property against a guide roll or the like. In order to solve such problems with handling of a film, a technology of including inert particles, such as inorganic particles, within a polyester has been used. Although the method of including inorganic particles or the like within a polyester includes a method of adding inorganic particles or the like at any stage in polyester polymerization and a method of including inorganic particles or the like in a previously produced polyester resin later by melt kneading, the addition in polymerization is preferred in terms of the dispersibility of inorganic particles. However, agglomeration of inorganic particles is an unavoidable problem even in the addition in polymerization. Therefore, various devices have been made to the properties of inorganic particles or the method of addition of particles in polymerization (see, for example, Patent Documents 8, 9 and 10). The above-cited patent documents have reported that as the best way for preventing agglomeration of inorganic particles, it is effective to apply surface treatment to the inorganic particles, further adding a polycondensation catalyst separately from the inorganic particles, and further adding the particles while dividing them in many portions.
The present inventors attempted to polymerize a polyester suitable for a film with little coarse particles derived from agglomeration of inorganic particles by using the technologies of polymerizing a polyester containing inorganic particles to be used for a film described in the above-cited Patent Documents 8, 9 and 10 and the polymerization catalyst technologies disclosed in the above-cited Patent Documents 1, 2, 6 and 7. As a result, a certain improving effect has been attained for the existing problems, but they recognized that it was insufficient for obtaining a further high value-added film. Specifically, it was found that when polymerizing a polyester using a polymerization catalyst made of an aluminum compound, polymerization with addition of inorganic particles in a prescribed amount necessary for imparting slipping property to a film resulted in agglomeration of the inorganic particles and defects that could not be disregarded as a high quality film were generated. In addition, it is recognized that deterioration in polymerization activity, deterioration in heat resistance, and change in color tone each caused by the addition of inorganic particles were problems that could not be disregarded.
In addition, the use of substances other than heavy metals such as antimony compounds, germanium compounds, or tin compounds, are desired as a polycondensation catalyst for a polyester from consideration for the environment. A titanium compound has been proposed as a substitute for such heavy metal-based polycondensation catalyst. However, a polyester produced using this is problematic in that the polyester is prone to thermal degradation in melt molding and the polyester is colored remarkably.
According to the above circumstances, there is a demand for a polyester composition using a polyester polycondensation catalyst including a metal component other than antimony compounds, germanium compounds, titanium compounds, and tin compounds, as a main component, the polyester composition being excellent in polymerization activity, causing little coloration or deterioration in heat resistance when inorganic particles are included therein, causing little generation of coarse particles due to agglomeration of inorganic particles, and being suitable for a high quality film application.